refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/pkg/trace/propagator.go

@@ -0,0 +1,15 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package trace 提供 OpenTelemetry 链路追踪封装工具
+package trace
+
+import "go.opentelemetry.io/otel/propagation"
+
+func newPropagator() propagation.TextMapPropagator {
+	return propagation.NewCompositeTextMapPropagator(
+		propagation.TraceContext{},
+		propagation.Baggage{},
+	)
+}

backend/pkg/trace/sampler.go

@@ -0,0 +1,19 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package trace
+
+import (
+	sdktrace "go.opentelemetry.io/otel/sdk/trace"
+)
+
+// ParentBasedRatioSampler 创建父级感知的概率采样器
+// - 如果父 Span 已采样，则子 Span 也采样
+// - 如果父 Span 未采样，则子 Span 也不采样
+// - 如果是根 Span，按 samplingRate 概率采样
+func ParentBasedRatioSampler(samplingRate float64) sdktrace.Sampler {
+	return sdktrace.ParentBased(
+		sdktrace.TraceIDRatioBased(samplingRate),
+	)
+}

backend/pkg/trace/trace.go

@@ -0,0 +1,62 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package trace
+
+import (
+	"context"
+	"log"
+
+	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/trace"
+)
+
+// Tracer 全局 OpenTelemetry Tracer 实例
+var Tracer trace.Tracer
+var shutdownFuncs []func(context.Context) error
+
+func init() {
+	// 初始化 Propagator
+	prop := newPropagator()
+	otel.SetTextMapPropagator(prop)
+
+	// 初始化 Tracer 实例为 No-op 默认以避免未初始化前或测试环境崩溃
+	Tracer = otel.GetTracerProvider().Tracer("github.com/Rain-kl/Wavelet")
+}
+
+// Config 链路追踪配置
+type Config struct {
+	AppName      string
+	SamplingRate float64
+	TracerName   string
+}
+
+// Init 初始化 Tracer Provider 并关联全局 Tracer 实例
+func Init(cfg Config) {
+	tracerProvider, err := newTracerProvider(cfg)
+	if err != nil {
+		log.Fatalf("[Trace] init trace provider failed: %v", err)
+	}
+	shutdownFuncs = append(shutdownFuncs, tracerProvider.Shutdown)
+	otel.SetTracerProvider(tracerProvider)
+
+	// 更新 Tracer
+	tracerName := cfg.TracerName
+	if tracerName == "" {
+		tracerName = "github.com/Rain-kl/Wavelet"
+	}
+	Tracer = tracerProvider.Tracer(tracerName)
+}
+
+// Shutdown 关闭所有 Trace Provider
+func Shutdown(ctx context.Context) {
+	for _, fn := range shutdownFuncs {
+		_ = fn(ctx)
+	}
+}
+
+// Start 创建一个新的 Trace Span
+func Start(ctx context.Context, name string, opts ...trace.SpanStartOption) (context.Context, trace.Span) {
+	return Tracer.Start(ctx, name, opts...)
+}

backend/pkg/trace/trace_provider.go

@@ -0,0 +1,52 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package trace
+
+import (
+	"context"
+	"os"
+
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc"
+	"go.opentelemetry.io/otel/sdk/resource"
+	sdktrace "go.opentelemetry.io/otel/sdk/trace"
+)
+
+func newTracerProvider(cfg Config) (*sdktrace.TracerProvider, error) {
+	// 获取主机名和容器信息
+	hostname, err := os.Hostname()
+	if err != nil {
+		return nil, err
+	}
+
+	// 业务属性不绑定 schema URL，合并时继承 resource.Default() 的 SDK 内置版本，避免 semconv 与 otel/sdk 升级不同步。
+	r, err := resource.Merge(
+		resource.Default(),
+		resource.NewSchemaless(
+			attribute.String("service.name", cfg.AppName),
+			attribute.String("host.name", hostname),
+			attribute.String("k8s.namespace.name", os.Getenv("KUBERNETES_NAMESPACE")),
+			attribute.String("k8s.pod.name", os.Getenv("KUBERNETES_POD_NAME")),
+			attribute.String("k8s.pod.uid", os.Getenv("KUBERNETES_POD_UID")),
+		),
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	// 初始化 Exporter
+	traceExporter, err := otlptracegrpc.New(context.Background())
+	if err != nil {
+		return nil, err
+	}
+
+	// 初始化 Trace
+	tracerProvider := sdktrace.NewTracerProvider(
+		sdktrace.WithBatcher(traceExporter),
+		sdktrace.WithResource(r),
+		sdktrace.WithSampler(ParentBasedRatioSampler(cfg.SamplingRate)),
+	)
+	return tracerProvider, nil
+}